1. Field of the Invention
The present invention relates to duplexers, and more particularly, to a duplexer having a frequency discrimination capability, a circuit structure of the duplexer and an RF (radio frequency) transceiver apparatus comprising the duplexer.
2. Description of Related Art
In current wireless communication systems, RF (radio frequency) apparatus are required to operate under multiple frequency bands. For example, duplexers are indispensable elements of multi-band communication systems and multiplex systems. A duplex has three ports. One of them is an input port and the other two are output ports for separating signals at different frequencies. Alternatively, two of the three ports are input ports and the other one is an output port for integrating signals at different frequencies.
FIGS. 1 and 1′ are respectively a schematic circuit diagram and a circuit layout diagram of a conventional duplexer 1.
Referring to FIGS. 1 and 1′, the duplexer 1 has a circuit structure 1′ formed on a substrate 1a. The duplexer 1 has: a first signal port 17; a first filter 10a electrically connected to the first signal port 17 through a first resonant circuit 11; a second signal port 18a electrically connected to the first filter 10a; a second filter 10b electrically connected to the first signal port 17 through a fourth resonant circuit 14; a third signal port 18b electrically connected to the second filter 10b through a fourth resonant circuit 14; and ground ports 19. The first filter 10a has a second resonant circuit 12 and a third resonant circuit 13 and a main capacitor 100. The second filter 10b has a fifth resonant circuit 15, a sixth resonant circuit 16 and a main inductor 101. The ground ports 19 are connected to the second resonant circuit 12 and the fifth resonant circuit 15, respectively.
The first resonant circuit 11 has a first inductor L1 connected in parallel with a first capacitor C1 and electrically connected to the first signal port 17.
The second resonant circuit 12 has a second inductor L2 connected in parallel with a second capacitor C2 and electrically connected to the first resonant circuit 11 in series.
The third resonant circuit 13 has a third inductor L3 connected in parallel with a third capacitor C3 and electrically connected to the second signal port 18a. 
The main capacitor 100 is connected in series with the second and third resonant circuits 12, 13 and located therebetween.
The fourth resonant circuit 14 has a fourth inductor L4 connected in parallel with a fourth capacitor C4 and electrically connected to the first signal port 17.
The fifth resonant circuit 15 has a fifth inductor L5 connected in parallel with a fifth capacitor C5 and electrically connected to the fourth resonant circuit 14 in series.
The sixth resonant circuit 16 has a sixth inductor L6 connected in parallel with a sixth capacitor C6 and electrically connected to the third signal port 18b. 
The main inductor 101 is connected in series with the fifth and sixth resonant circuits 15, 16 and located therebetween.
In the duplexer 1 of FIG. 1, the inductors and capacitors have inductances and capacitances as follows.
CapacitorCapacitanceInductorInductanceC10.7015 pFL11.1224 nHC23.8487 pFL21.1787 nHC32.9604 pFL32.9604 nHC41.6818 pFL42.6941 nHC5 1.403 pFL50.5612 nHC60.8684 pFL60.5786 nHMain capacitor 1000.5787 pFMain inductor 1012.5465 nH
Generally, a full lumped element method is used for the design of the duplexer 1 and accordingly two electrical matching networks and two filters 10a, 10b are needed to separate signals at different frequencies sent from the first signal ports 17, and the separated signals are further transmitted through the second and third signal ports 18a, 18b, respectively. The two electrical matching networks are the first and fourth resonant circuits 11, 14.
To achieve the above-described design of the duplexer 1, the first, second, third, fourth, fifth, and sixth resonant circuits from 11 to 16, the main inductor 101 and the main capacitor 100 need to be formed on a same surface of the substrate la. As such, a large sure area of the substrate la is occupied by the circuit structure 1′ of the duplexer 1, thus hindering miniaturization of the semiconductor element.
Further, since signals from the first signal port 17 are transmitted into the first and fourth resonant circuits 11, 14, respectively, it results in a poor frequency discrimination capability for the duplexer 1, which will be described later.
Therefore, how to overcome the above-described disadvantages has become urgent.